This invention relates to belt drive systems for vehicles and, more particularly, to a two belt drive system for lawn and garden tractors and other vehicles equipped with variable speed transmission/transaxle assemblies which include two rotational inputs and allow variation in ground speed while maintaining a constant engine drive speed.
Recent designs of variable speed transmission/transaxle assemblies for use with lawn and garden tractors and other small vehicles allow tractor ground speed to be varied from forward through neutral or zero output and reverse while maintaining tractor engine speed at a constant rate. Maintenance of constant engine speed enables constant speed rotation of mower blades for better, more effective cutting while prolonging engine life by reducing wear on the engine. One such variable speed transmission/transaxle is disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 06/759,679 filed July 29, 1985, entitled VARIABLE SPEED TRANSAXLE ASSEMBLY invented by Roland von Kaler and Rocky H. Page, now U.S. Pat. No. 4,726,256. Such transaxle incorporates a planetary gear assembly requiring separate, continuously operated, rotational drive inputs. One is driven at a constant rotational speed. The rotational speed of the other is varied to change the ultimate output speed of the transmission/transaxle and thus the tractor gound speed. In order to operate such a transmission/transaxle, while maintaining a constant engine speed, a three belt drive system is disclosed in Ser. No. 06/759,679 including two belts which are driven directly by the engine drive shaft and a third belt which is driven by one of the two drive belts through a variable pulley assembly.
The above type of three belt drive system is relatively expensive because of the inclusion of the number of belts and necessary supporting pulley components, tensioners and the like. Use of such a three belt system for both pretensioned or taut belt systems and passive belt systems, which operate with less tension on the belts but incorporate additional tensioning elements for slack take-up, has been difficult because of the necessity of tensioning or taking up the slack in three separate belts. With the normal wear encountered in all belt systems, replacement of three separate belts is more time consuming, expensive and difficult, especially if the system is pretensioned or taut where loads on the belts are relatively high.
An additional factor which must be considered in operating a variable speed transmission/transaxle assembly of the type described in U.S. Pat. No. 4,726,256 above is the control of the belt system to allow return of the system to neutral or zero transmission output when the vehicle brakes are applied. Also, provision must be made to allow pushing or movement of the tractor when the engine is shut off. Each of these operations is significant in a variable speed, planetary type transmission where gearing is constantly engaged and belt drives are constantly operating even when the transmission output is zero or neutral. Since the belts in such a drive system are always in driving tension in order to constantly rotate both inputs of the transmission/transaxle, movement of the vehicle when the engine is off would typically require rotation of all the drive belts against the compression of the engine cylinders.
Accordingly, the need was apparent for a reliable, less expensive belt drive system for lawn and garden tractors and other vehicles of the type incorporating planetary and other variable speed, dual input transmission/transaxle assemblies and adequate for use in both pretensioned or taut belt systems and in passive belt systems. The system must operate in both heavy and light load ranges, allow control for return to neutral or zero output when vehicle brakes are applied, and allow pushing or movement of the vehicle when the engine is off and the belts are not rotating. The present belt drive system was devised in recognition of and as a solution for these requirements.